1. Field of the Invention
The present invention relates to an X-ray imaging tube having a high resolution.
2. Description of the Related Art
Conventional X-ray imaging tubes have been generally used in a variety of applications as medical X-ray image pickup apparatuses and industrial non-destructive testing X-ray TV monitors.
A conventional X-ray imaging tube comprises a vacuum envelope having an input window for receiving an X-ray. An arcuated substrate is arranged inside the vacuum envelope to oppose the input window. An input phosphor screen and a photocathode are sequentially stacked on the opposite surface of the arcuated substrate with respect to the input window. A focusing electrode is arranged along the inner side wall of the vacuum envelope. An anode and an output phosphor screen are arranged on the output side.
An X-ray emitted from an X-ray tube passes through an object to be examined and then passes through the input window and the substrate of the X-ray imaging tube. The X-ray is then converted into light by the input phosphor screen. This light is converted into electrons by the photocathode. The electrons are accelerated and focused by an electron lens constituted by the focusing electrode and the anode. The focused electrons are converted into a visible image by the output phosphor screen. This visible image is picked up by a television camera, movie camera, or spot camera and is used for medical diagnosis.
An arrangement of the input phosphor screen of the X-ray imaging tube will be described with reference to FIG. 1.
Referring to FIG. 1, the input phosphor screen comprises an aluminum substrate 1, a discontinuous layer 2 made of cesium iodide (CsI) formed on the aluminum substrate 1, a continuous layer 3 made of cesium iodide (CsI) formed on the discontinuous layer 2, and a photocathode 4 formed on the continuous layer 3. The input phosphor screen having the above structure has a light guide effect. That is, since cesium iodide has a refractive index of 1.84 for emission at a wavelength of about 420 nm, light emitted by the cesium iodide crystal is theoretically subjected to total reflection when it is incident on an interface between the crystal and the vacuum at an obtuse angle of 33x or more. For this reason, the light cannot emerge outside the crystal. Part of emission cannot be scattered laterally and reaches the photocathode 4.
The light is attenuated at the interface between the crystal and the vacuum. Light emerging outside the crystal at a critical angle of 33.degree. or less reaches the adjacent discontinuous layer 2. At the time, most of the light is absorbed by the adjacent discontinuous layer 2, but the light partially returns to the original crystal by Fresnel reflection. This applies to emergence of light from the crystal to the vacuum. Light scattered laterally is gradually attenuated. Light farther away from a crystal growth direction passes the interface more frequently, thereby increasing the degree of attenuation. Therefore, light closer to the crystal growth direction can reach the photocathode 4 with a small attenuation amount.
Light emerging from the discontinuous layer 2 reaches the photocathode 4 which is not far away from a light emission point. A resolution of the input phosphor screen itself is thus obtained. Since a recent X-ray imaging tube aims at detecting X-ray signals passing through the object as much as possible, the thickness of the input phosphor screen is set to be 400 .mu.m or more, thereby improving X-ray absorption efficiency.
The light guide effect does not depend on the thickness of the input phosphor screen. When the thickness of the input phosphor screen, however, is increased, a light attenuation effect at the interface between the vacuum and the crystal is weakened, and the resolution of the input phosphor screen is decreased.
In order to increase this resolution, it is possible to reduce the diameter of each columnar crystal of the discontinuous layer 2 to obtain a dense optical interface in the planar direction. It is assumed that the dense optical interface increases the light attenuation rate (per unit optical length) of the laterally scattered light.
The diameter of each columnar crystal of the discontinuous layer 2 depends on the substrate temperature in a screen deposition process. When a cesium iodide film was formed at a pressure of 4.5 Pa while the substrate temperature was maintained at 150.degree. C. during deposition, a discontinuous layer 2 of columnar crystals each having a diameter of 6 fm was obtained. When the substrate temperature was set at 180.degree. C., a discontinuous layer 2 of columnar crystals each having a diameter of 9 .mu.m was obtained. When the resolutions of input phosphor screens having these discontinuous layers 2 were measured, CTF (Contrast Transfer Function) values of these samples were almost equal to each other, about 24% at 20 lp/cm. The CTF value of the input phosphor screen having the discontinuous layer of columnar crystals each having the diameter of 6 .mu.m was larger than that of the columnar crystals each having the diameter of 9 .mu.m by 1% at 50 lp/cm. This CTF difference results in a small difference appearing on the TV monitor through an image pickup system when the input phosphor screen is mounted in an X-ray imaging tube.
As another effective means for improving resolution characteristics of an input phosphor screen having such a columnar structure, a light-absorbing or light-reflecting layer is formed at the optical interface constituted by the columnar structure, thereby increasing the lateral light attenuation. In particular, a method of increasing light attenuation at the interface between the crystal and the vacuum is disclosed in Published Unexamined Japanese Patent Application No. 62-43046. According to this method, a light-absorbing layer is formed between the crystal columns of the discontinuous layer. Another method is disclosed in Published Unexamined Japanese Patent Application No. 59-121733, in which a light-reflecting material powder is filled between the columns of the discontinuous layer. However, since the gap between the columns of the discontinuous layer is 1 .mu.m, it is very difficult to perform the above process in the small gap between the crystal columns.
To the contrary, Published Examined Japanese Patent Application No. 54-40071 describes that a columnar phosphor mixed with copper is annealed in an oxygen atmosphere to form an oxide film at the optical interface of the columnar phosphor, thereby obtaining an input phosphor screen. This prior art describes that light within the phosphor is reflected by the oxide film on the input phosphor screen and will not emerge outside the phosphor.